Polyketides are complex natural products that are produced by microorganisms such as fungi and mycelial bacteria. There are about 10,000 known polyketides, from which numerous pharmaceutical products in many therapeutic areas have been derived, including: adriamycin, epothilone, erythromycin, mevacor, rapamycin, tacrolimus, tetracycline, rapamycin, and many others. However, polyketides are made in very small amounts in microorganisms and are difficult to make or modify chemically. For this and other reasons, biosynthetic methods are preferred for production of therapeutically active polyketides. See PCT publication Nos. WO 93/13663; WO 95/08548; WO 96/40968; WO 97/02358; and WO 98/27203; U.S. Pat. Nos. 4,874,748; 5,063,155; 5,098,837; 5,149,639; 5,672,491; 5,712,146 and 6,410,301; Fu et al., 1994, Biochemistry 33:9321-26; McDaniel et al., 1993, Science 262: 1546-1550; Kao et al., 1994, Science, 265:509-12, and Rohr, 1995, Angew. Chem. Int. Ed. Engl. 34: 881-88, each of which is incorporated herein by reference.
Biosynthesis of polyketides may be accomplished by heterologous expression of Type I or modular polyketide synthase enzymes (PKSs). Type I PKSs are large multifunctional protein complexes, the protein components of which are encoded by multiple open reading frames (ORF) of PKS gene clusters. Each ORF of a Type I PKS gene cluster can encode one, two, or more modules of ketosynthase activity. Each module activates and incorporates a two-carbon (ketide) unit into the polyketide backbone. Each module also contains multiple ketide-modifying enzymatic activities, or domains. The number and order of modules, and the types of ketide-modifying domains within each module, determine the structure of the resulting product. Polyketide synthesis may also involve the activity of nonribosomal peptide synthetases (NRPSs) to catalyze incorporation of an amino acid-derived building block into the polyketide, as well as post-synthesis modification, or tailoring enzymes. The modification enzymes modify the polyketide by oxidation or reduction, addition of carbohydrate groups or methyl groups, or other modifications.
In PKS polypeptides, the regions that encode enzymatic activities (domains) are separated by linker regions. These regions collectively can be considered to define boundaries of the various domains. Generally, this organization permits PKS domains of different or identical substrate specificities to be substituted (usually at the level of encoding DNA) from other PKSs by various available methodologies. Using this method, new polyketide synthases (which produce novel polyketides) can be produced.
It will be recognized from the foregoing that genetic manipulation of PKS genes and heterologous expression of PKSs can be used for the efficient production of known polyketides, and for production of novel polyketides structurally related to, but distinct from, known polyketides (see references above, and Hutchinson, 1998, Curr. Opin. Microbiol. 1:319-29; Carreras and Santi, 1998, Curr. Opin. Biotech. 9:403-11; and U.S. Pat. Nos. 5,712,146 and 5,672,491, each of which is incorporated herein by reference).
One valuable class of polyketides are the disorazoles. Disorazoles are a family of complex 26-membered bislactone macrocycles having two oxazole rings, which were first detected in the So cel2 strain of Sorangium cellulosum (Irschik et al., 1995, The Journal of Antibiotics, 48:31-35). The So cel2 strain produces 29 congeners of disorazole compounds, with disorazole A (1) being the predominant product (see structure 1, below, and FIG. 1).

Disorazole A shows remarkable activity against eukaryotic cells, having high mammalian cell cytotoxic activity (MIC ˜3-30 pg/ml) and activity against different fungi, including filamentous fungi belonging to the Ascomycetes, Basidiomycetes, Zygomycetes, Oomycetes, and Deuteromycetes families (MIC ˜0.1-1 μg/ml). In contrast, the compound is not highly active against yeast and bacteria. Jansen et al., 1994, Liebigs Ann. Chem., pp. 759-73.
The present invention provides polynucleotides and methods for biosynthesis of disorazoles, disorazole derivatives, and novel polyketides.